Speaker systems are widely used both commercially and personally for projecting or reproducing various sounds, principally music and voice. In the field of high fidelity music reproduction, substantial effort has been expended in an attempt to provide optimum sound reproduction. In particular, the most difficult area is the reproduction of lower frequencies or bass sounds, with bass sound reproduction in small or undersized loudspeaker enclosures being the most difficult.
In an attempt to satisfy this need, many methods and systems have been developed seeking to provide extended or enhanced bass response, using both single and multiple drivers. The prior art methods systems and constructions include tuned parts, ducted parts, vented ports, bass reflex, labyrinth, horn, folded horn, corner reflection, acoustic suspension, air suspension, asymmetric signal generation, high compliance and low frequency drivers, and electronic bass boosters. Regardless of this substantial effort, the desired goal has remained unsatisfied.
Each of these prior art attempts rely upon a tuned, undamped resonant acoustical path, acoustical circuit, or intrinsic enclosure resonances to enhance the loudspeaker system""s efficiency at the desired bass frequencies or frequency bands. However, these systems suffer from several common shortcomings.
Firstly, all known resonant tuning methods create an acoustical path of little or no resistance or acoustical impedance, which in turn, causes a very high acoustical efficiency for the frequencies or frequency bands of concern. This results in a lack of critical acoustical damping of the reproduced signal. Critical damping is commonly known as xe2x80x9cunity dampingxe2x80x9d.
Any value for damping, in a loudspeaker system, which is significantly more or less than unity is not suitable for the accurate reproduction of sound. Systems utilizing the prior art types of tuning will usually generate the required amount of bass frequency output. However, the quality of the acoustic output will have little or no relationship to the original input signal. They alter or xe2x80x9ccolorxe2x80x9d the signal to effect an artificial bass sound.
Another problem usually encountered with these prior art systems, typically due to the insufficient damping, is a condition called xe2x80x9cringingxe2x80x9d. Ringing is typified by the condition where the diaphragm of a loudspeaker or driver is not under control of the applied signal. As a result, abnormally high levels of distortion are produced, along with the generation of false or xe2x80x9ccoloredxe2x80x9d acoustical signals when compared to the original input signal.
Basically, ringing is produced by the excitation of the driver""s moving mass, the speaker cone and/or voice coil and suspension, by the electronic signal that causes the mass to oscillate at a fundamental period which is not necessarily related to the original input signal. The fundamental period, frequency or musical note, is an acousto-mechanical product of the total moving mass, the stiffness of the air within the enclosure, the compliance of the driver""s suspension, the length of the resonant path and other associated variables. A ringing condition is frequency selective, and therefore, not faithful to the original input signal. A ringing system can usually be excited into the ringing mode by a frequency totally unrelated, but close to the ringing or resonant frequency.
The problems typically found in tuned, resonant, undamped and ringing prone systems are very high levels of acoustical distortions. Typical types of acoustical distortion produced by these systems are harmonic distortion or total harmonic distortion, intermodulation distortion, and phase distortion.
Harmonic distortion or total harmonic distortion is the type of distortion which is generated in several various ways. However, in the systems detailed herein, harmonic distortion is classified by its incidental compositionxe2x80x94second order (first harmonic), third order (second harmonic), fourth order (third harmonic). For example, a 100 Hz fundamental signal generates a 200, 300, and 400 Hz frequency. Sub-harmonics may also occur at half, one-third, one-quarter, etc. of this fundamental note. Under the condition of ringing in an undamped, resonant system, it is natural for the loudspeaker to produce these coincidental notes or frequencies.
Intermodulation distortion is more technically involved and consists of products totally unrelated or not incidental to the original frequency. Typically, intermodulation distortion is generated by the remodulation of one signal by another which in turn produces sum and difference frequencies. If, for example, the air pressure within a loudspeaker enclosure is at a very high level for one particular frequency as compared to a lower pressure at another frequency, the higher pressure condition will tend to modulate the lesser pressure condition thereby creating a sum product frequency and a difference frequency. These sum and difference notes do not exist in the original input signal, thus, they are unwanted distortions.
Finally, phase distortion is produced when highly resonant, undamped ringing occurs in the xe2x80x9ctime domainxe2x80x9d. Essentially, from the initial excitation, until the ringing subsides, a finite period of time passes. Since the ringing loudspeaker is producing sound long after the original input electrical signal is gone, the moving mass of the loudspeaker""s driver continues to oscillate at a rate or period based on its own physical characteristics. This effect generally causes the ringing frequency to either increase or decrease in its fundamental period. Since the original input frequency was of a given period with its own specific phase relationships, any variation from the original will, of consequence, create different, unrelated phase components.
In addition, the products of total harmonic distortion and intermodulation distortion, either together or individually, will mix acoustically and/or electrically in multiplicities to create very complex, albeit unwanted, by-products. Although this condition is most easily identified and analyzed as additional total harmonic distortion and intermodulation distortion, subsequent additional phase distortion will always result.
By employing the present invention, all of the difficulties and drawbacks of prior art loudspeaker constructions are overcome, and a loudspeaker system is realized which is able to faithfully reproduce desired input signals, particularly, lower frequencies or bass sounds. Furthermore, this result is achieved in a seemingly small or undersized enclosure which is able to exhibit a linear loading coefficient and unity coupling factor for the wavelengths or frequencies of concern.
In accordance with the present invention, the enclosure is physically smaller than seemingly required for providing actual enclosures air volume for optimum performance of the loudspeaker. However, using the present invention, the enclosure has an acoustically optimum size, providing an effective or virtual air volume for optimum performance. In addition, the loudspeaker of the present invention eliminates the distortion components found in prior art systems.
As is well known, the ideal condition for the essentially distortion-free driver enclosure is the xe2x80x9chalf-spacexe2x80x9d baffle. In a half-space baffle, the shortest dimension is greater than a half-wavelength of the lowest frequency that the driver is capable of producing. As a result, the front wave is incapable of interacting with the back wave.
In order to achieve this result, the hypothetical half-space baffle must be infinite in length and width. Since this is physically impossible, reasonable compromises must be made.
Any loudspeaker enclosure that isolates, then dampens, and finally absorbs or otherwise dissipates the back waves of a driver from the front waves and is of suitable size to prevent reactive air pressure levels or otherwise impede the operation of the driver, is called an xe2x80x9cinfinite bafflexe2x80x9d. Under these ideal conditions, the enclosure is infinite in volume, as far as the driver is concerned. However, since a bass driver of a given size and of any given electrical-acoustical-mechanical characteristics require an enclosure of a specific volume for optimum performance, anything less in internal enclosure volume yields less than optimal results.
In attempting to achieve the ideal enclosure construction, the prior art enclosures are generally constructed with opposing surfaces that are parallel, flat, and dimensionally constant at any perpendicular position; a rectangular box. However, by totally deviating from these standard configurations, a new, unique, highly effective speaker enclosure is realized.
In particular, the loudspeaker enclosure of the present invention is cylindrical in cross-section, and comprises the loudspeaker or driver mounted at one end of the cylinder.
In addition, the opposed or rear end of the cylindrical enclosure is sealed with a hemispherical baffle which is positioned to have a concave shape when viewed from the outside. As a result, internally, the baffle forms a convex shape to provide a rigid surface for the acoustical pressures generated by the speaker""s moving diaphragm or cone.
In addition, in accordance with the present invention, a plurality of holes or vents are formed in the rear baffle. These vents are constructed with a diameter and position which effectively forms a plurality of acoustical paths which are non-coincidental in wavelength dimension along the axial dimension of the enclosure.
As a result, the internal volume of air within the enclosure of the present invention will not be resonant at any one specific frequency. This causes the enclosure to be effectively or virtually acoustically infinite in volume.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.